Method of separating nitrogen isotopes by ion-exchange



United States Patent METHOD OF SEPARATING NITROGEN ISOTOPES BYION-EXCHANGE Frank H. Spedding and Jack E. Powell, Ames, Iowa, as-

signors to Iowa State College Research Foundation, Inc., Ames, Iowa, acorporation of Iowa No Drawing. Application February 29, 1956 Serial No.568,438

4 Claims. (Cl. 23-193) This invention relates to a method of separatingnitrogen isotopes by ion-exchange.

It has been known heretofore that some degree of separation betweenisotopes of a given element could be obtained by means of ion-exchangeprocesses. Actually, with respect to prior art processes, enhancementwould be a more accurate term than separation. For example, Patent No.2,204,072, issued June 11, 1940, describes the use of ion-exchangeprocesses to change the ratio of isotopes of an element. Nothingapproaching actual separation of isotopes is described or claimed.Typical of the results obtained is the decrease by ten percent of therare nitrogen isotope N in a solution of ammonium chloride by passing itthrough a column of sodium zeolite.

It is therefore a general object of this invention to provide animproved method of separating nitrogen isotopes by ion-exchange. Furtherspecific objects are to provide a process of the character describedwhich permits nitrogen isotopes to be produced in a highly purified formby a simple and efficient procedure. Further objects and advantages willbecome apparent as the specification proceeds.

The ion-exchange process of the present invention is distinguished fromprior ion-exchange processes for isotope separation in that chemicalconstraints are :em- 'ployed at both the front and rear boundaries of.the adsonbed band as it is displaced through the ion-exchange material.These chemical constraints serve-the purpose of maintaining sharpboundaries to define a band of substantially fixed length, whereby trueisotopic equilibrium can be closely'approached within the hand eventhough the isotopic exchange constants are'very small. -Morespecifically, chemical reactions should occur at the front and rearboundaries of the migrating band which have equilibrium constants of theorder of 10 or greater. -In other words, if a boundary is to remainsharp, a chemical reaction which goes virtually to completion must takeplace at the boundary.

In practicing the method of this invention, 'an aqueous solution ofammonium hydroxide is passed through. a portion of a cation exchangematerial in the'hydrogen cycle to provide an adsorbed ammonium bandhaving -a sharply-defined front boundary, 'It will be understood thatthe ammonium hydroxide will provide both N and N ammonium ions. Theadsorbed ammonium is then eluted and the band i'sdisplaced to successiveportions of a cation exchange material in the hydrogen cycle by passingan aqueous solution of an alkali metal "hydroxide through the cationexchange 'material behind the band. This maintains a sharply-definedrear bound- '-ary for the band. The eluting and displacing procedure iscontinued untilthe rear portion of the band contains ani'app'reciablyhigher ratio 0f1N ammonia to. N am- -monia than the front portion ofthe.band.- Preferably, ;it is continued until ,the .rearportionv of theband contains of the ammonia in the rear portion. The rear and 2 frontportions of the band can then be separated, thereby producing productscontaining the separated isotopes.

Any cation exchange material which can 'be placed in the hydrogen cycleand maintained therein can be commercially available cation exchangeresins of the,

strong acid type are: Amberlite IR-120 (Rohm & Haas, Washington Square,Philadelphia 5, Pennsylvania), Dowex-SO (Dow Chemical, Midland,Michigan), Permutit Q (Permutit, 330 W. 42nd Street, New York 18,

New York), Duolite C 20 (Chemical Process, Redwood City, California),Amberlite IR-lOO (Rohm & Haas), Zeo-Rex (Permutit), and Duolite C-3(Chemical Process). The last three in the foregoing list are of thephenolic methylene sulfonic-type, while the first four are of thenuclear sulfonic, polystyrene base-type. Other cationic resins includethe carboxylic resins like Amberlite IRC50 (Rohm & Haas), Duolite CS-lOO(Chemical 25 Process), and Permutit 216 (Permutit).

in the hydrogen cycle. can be placed in the hydrogen cycle according tousual procedures. the column, it can be washed with a dilute mineral'acid such as 1 N H 80 be continued until the exchange material issaturated,

As indicated, the cation exchange material is used It will be understoodthat it For example, after the resin is packed into This washing willusually and then it will be Washed with plain water, preparatory -to theintroduction of the ammonium hydroxide solution.

As already indicated, the mixture of nitrogen isotopes to be separatedis introduced into the ion-exchange columns in the form of an aqueoussolution of ammonium hydroxide. The concentration of the am moniumhydroxide solution is not particularly critical, "although it ispreferred to use relatively dilute soluof an alkali metal hydroxide,such as sodium or potas- 50 -sium hydroxide.

-cal For example, the normality of the alkali metal hydroxide solutioncan range from about 0.01ynorma1 to 3.0 normal.

" When this invention is practiced using the reagents and procedure justdescribed, the adsorbed band of ammonia will have a sharp front due 'tothereaction:

A dilute hydroxide solution is preferred, although the concentration isnot particularly criti- I To translate the above equation into verbalterms, the hydrogen on the resin interacts with the ammonium hydroxideto provide ammonia ions on the resin and water as areaction product.this reaction is of the order of 10 it can be seen that ;the conditionsfor an extremely sharp boundary are met. Similarly, the sharp rearboundary for the absorbed ammonium band is maintained by a chemicalreaction, which when the eluant is sodium hydroxide, can be illustrated.by the equation:

aa e 7 m e ce ..qf.. Nli.amm nia 9. theba s:

Since the equilibrium constant of The above equation showsthat theammonium ions on the resin are exchanged for the sodium ions in thedroxide.

eluant, and that the free ammonium ions then react with the hydroxylions to form ammonium hydroxide. The equilibrium constant of thisreaction is of the order of 10 which again produces a sharp boundary.

Under the equilibrium conditions just described, separation of thenitrogen isotopes is greatly accelerated by the reactions occurringrespectively at the front and rear boundaries of the adsorbed ammoniumband. This acceleration can be explained by the fact that a molecule(ammonium hydroxide) of substantial stability is formed in the solutionso that this molecular form in effect competes with the resin for theammonium ion. Thus, in addition to the exchange reaction between theresin in the solution, which is indicated by Equation 1 below, there isan exchange reaction occurring within the solution itself, as indicatedby Equation 2 below:

In the above equations, the subscript S indicates solution, while thesubscript R indicates the resin. While the nitrogen isotope separationprocess of the present invention involves other factors of importance,it is believed that the equilibrium expressed in Equation 2 is ofcritical importance for the success of the process.

It will be understood, of course, that in order to develop and maintaina sharp adsorbed ammonium band, it is necessary to have a uniformlypacked resin bed. Under these conditions, as the solution percolatesdown the column, a horizontal boundary between the two solutions can bemaintained. In practice, if sufiicient care is talcen in preparing theresin beds, tilting and channeling can be kept less than a fewmillimeters for columns two to six inches in diameter. Under suchconditions, theadsorbed ammonium band is, accordingly, sharply confinedbetween two boundaries and is in equilibrium throughout its length witha solution of ammonium hy- The concentration of ammonium hydroxidesolution in contact with the ammonium band is constant throughout and isdetermined by the concentration of primarily determined by the amount ofammonia in the column and the capacity of the resin. The band lengthwill depend to a certain extent upon the concentration of the ammoniumhydroxide in the resin pores, which is fixed by the concentration of theeluant. Once the concentration of the alkali metal hydroxide has beenchosen, the length of the ammonium band remains substantially constantas it is eluted down the resin bed.

Since the length of the ammonium band remains constant'as the band iseluted down the resin bed, each time anequivalent of alkali metal ion isdeposited at the rear ofthe band it displaces an equivalent of ammoniumion. The displaced ammonium ion comes to chemical equilibrium within thesystem very rapidly and isotopic equilibrium tends to be approached asthe solution passes over theresin. When the ammonia in solution reachesthe front edge of the ammonium band, it is redeposited as ammonium ionin the resin bed. At all times, the ratio of N to N of the'resin isgreater than it is in the solution containing it. It has been found thatthe isotopic exchange for the system proceeds with suificient rapiditythat equilibrium is attained at the critical points in the system evenWhenthe band is moving at the rate of 30 inches per hour down a bed. Theband willget richer in N at the rear edge and richer in N at the frontedge as theelution proceeds. Between the front and rear portions of thehand, there tends to be a plateau'region where the ratio of the isotopesin the resin phase and solution phase are not appreciablychangingalthough they differ from each other. Experience has'shown thatit is only 'necessary to have a band of ammonia seven to :ten feet longin order to provide a suitable plateauregion between columns.

and sul'ficient distance on either side to permit the concentration ofthe isotopes to approach 100% N at the front and 100% N at the rear.After the ammonium band is adsorbed on the bed, it is eluted for adistance of 200 to 300 feet. This allows the system to approach acondition where continuous operation can be started. Feed solution canthen be injected at the top of each column, where a plurality of columnsare used, whenever the plateau region moves past the point of injection.Similarly products can be removed as the front and rear edges of theammonium band pass the bottom of a column. In practice, since lessvalving i required, the feed is injected and the products withdrawn onlyonce each time the band completes a circuit of the beds. In thisprocedure, a number of short columns connected in series can be usedrather than a single long column. As already indicated, this arrangementpermits an adsorbed band to be eluted in a cyclic manner using only afew columns, since the spent resin can be regenerated for reuse beforethe band comes around the next time. Other advantages of such anarrangement are simply to maintain uniform resin beds in short columns,and the resistance to .flow of the eluant .and regenerant solutions canbe minimized by having the solutions flow only through the desiredportion of the bed system in any given time. If it is desired to obtainthe maximum rate of separation of'isotopic species from a given feedmaterial, one strives for the maximum mass transfer of the isotopes andadjusts the system so that an effectivelycontinuous operation can becarried out.

Since the. natural abundance of N is low and the :resin bed has anappreciable capacity, it is desirable to permit the N to accumulatewithout withdrawal until its concentration has built up to the desiredlevel. Withdrawal of N can begin at the time the feed injections arestarted. .As'the ammonium band is eluted from one resin bed with-sodiumhydroxide eluant or other alkali metal hydroxide, the front section isallowed to proceed down the-next column until the plateau region ispassing At this time, a valve between the two columns is closed and aquantity of ammonium hydroxide is allowed to flow into the top of theleading column.

At the same time, or at some prior or subsequent time,

ammonium hydroxide containing virtually pure N is allowed to flow themthe bottom of the column. In practice, the withdrawal can be madeslightly before or after the :injection so as to occur when the front ofthe i=bandis moving off a column.

There are certain other general considerations which maybe mentioned asof interest to those who wish to .practice the-process of thisinvention. These considerations, however, relate mainly to theefiiciency, as distinguished from the operability of the process. More 7specifically, for most efficient operation it is desirable that: (1) theexchange reaction employed should have as large an equilibrium'constautas possible; (2) the band should be eluted as fast as practical toobtain the maximum; transport per unit time, but not so fast thattheband boundaries become ditfused or the :HETP excessively ."large; (3)the material should be .eluted under conditions which permitmaximumutilization of the active points on the resin bed by the ions beingseparated isoitopically; and.(4) the. length of vtheadsorbed band should"bEiSBlGCtt-Zd so .thatisufficient plates are included within the bandto .give the desired products at a :steady state, but

not :so long that excessive resistance to flow is encountered.

In one specific example, a bank of four inch by five monium band couldbe eluted around the bank of columns for as many band displacements asdesired. After traveling fifty band lengths, most of the N wasconcentrated in the last six inches of the band. The isotopic ratio, N-N of the original adsorbed ammonia was 0.00365. After fifty banddisplacements, the ratio at the front edge of the band usually droppedto 0.00020 and in some experiments to as low as 0.00006. The first 90%of the ammonia recovered had an average isotopic ratio less than0.00050. The isotopic ratio at the rear edge averaged better than 0.25.When the last 2% of the ammonia from such a run was adsorbed on oneinchcolumns and eluted, an additional 100 feet, the mole percent of N at therear edge exceeded 74%. Good results were obtained at flow rates whichproduced a band movement anywhere from a fraction of an inch to 30inches per hour, I

, Thisinvention is further illustrated'by the following detailedexperimental examples;

EXAMPLE I A series of ten ion-exchange columns were prepared. Eachcolumn consisted of a 5-foot section of 4-inch, I.D., flanged Pyrex pipeclosed at the ends by type 316 stainless steel plates bolted to standardcast iron flange sets. The end plates of the columns were center-tappedand fitted with 41-inch stainless steel nipples, two inches long. Acircular baflle plate, two inches in diameter, supported by three/2-inch long rods of stainless steel, was welded to the bottom side 'ofeach top plate to prevent the infiuent solution from disturbing theresin bed. The resin bed was supported by one thickness of Saran filtercloth backed up by a SO-mesh stainless steel screen. The circles ofSaran and stainless steel gauze were rubber-cemented into a 5-layersandwich between three 6-inch, O.D., 4-inch, I.D., by Vs-inch thickneoprene gaskets. The screen sandwich was supported an inch above thebottom plate by a Pyrex pipe spacer to prevent the screen from blockingthe outlet in the bottom plate. Neoprene gaskets were used with both thecolumn plates to obtain a water-tight seal. Individual columns of theseries were interconnected by means of flexible -inch Tygon tubingslipped over the Aa-inch pipe nipples which were screwed into the endplates. By using a binding of Scotch electrical tape over the junctionsand /z-inch brass tubing clamps, in addition, the system could be usedup to an internal pressure of two atmospheres without leakage.

The resin beds of the columns consisted of 100-200 mesh spheres of Dowex50-X12 and were 58-59 inches long in the H+ cycle and 53-54 inches longin the Na+ cycle. The resin beds were backwashed by removing the topplate and adding an auxiliary S-foot section of Pyrex pipe. Allexceptionally fine particles, which either did not settle readily afterbackwashing or which settled in a thin layer at the top of the resinbeds, were removed by means of a siphon so that the finished bedsoifered a minimum of resistance to liquid flow. Under a 60-foot liquidhead a flow rate of about a liter per minute could be obtained through asingle unit.

The resin beds were given a preliminary treatment with 2 N NaOH, rinsedwith distilled water and restored to the H+ cycle with l N H 80 Theexcess H+ ion was rinsed from the columns by means of distilled water,and five liters of 15 N NH OH, diluted 30-fold with water, was passed'downflow through three of the columns connected in series. Thisresulted in an adsorbed NH band approximately 10 feet long.

The ammonium band was eluted down the series of columns with 0.6 N NaOHat a flow rate of 120-180 ml. per minute. As the band progressed downthe series, the eluant feed tube and the discharge tube were moved alongso that only three columns were connected together at any one time. Asthe band passed, the resin left behind was reconverted from the Na+ tothe H+ cycle, as before, and the columns were then ready to be reused.In all, the adsorbedband was eluted around the l0-column series tentimes. Profiles were taken every twenty columns by withdrawing smallsamples of the solution periodically as the band passed. betweencolumns. It was found that the normal plateau is gradyond this, theplateau disappeared and the efliciencyfell oft markedly. As the bandapproaches the steady state, very little further improvement could beexpected. However, the last 6 inches of the band containing practicallyall the N was allowed to pass'onto a series of 22-mm.',' I.D., by 4%-foot resin beds and was eluted further until a mole fraction of N edgeof the band.

. EXAMPLE II Since normal ammonia contains only one part of N to 273parts of N enriched ammonia from several previous runs was adsorbed ontoa 2-inch diameter resin bed to form a band 45 inches long for thefollowing experiments. The band was eluted down 60 Z-inch by 5- footcolumns of Dowex 50-X12, -200 mesh 'resin, at a flow rate of ml. perminute with 0.6 N NaOH solution. This was more than suflicient to attaina steady state. The last 23 inches of this band, containing most of theoriginal N adsorbed on the column, was next eluted with the same NaOHsolution down 30 additional columns at one-fifth the original flow rate(25 ml. per minute). Since the length of the band was less, a shorterdistance of elution was required to ensure the attainment of steadystate conditions.

The last 14 inches of this band were then eluted down 10 additionalcolumns at the original flow rate of 125 ml. per minute, but withone-fifth the original concentration of NaOH, i.e., 0.12 N. Finally, thelast 5 /2 inches of the band from the previous experiment were eluteddown 3 more columns with 0.12 N NaOH at 25 ml. per minute. The data forthese various runs is summarized below in Table I.

Table 1 Flow Normality Rate (mL/min.)

of base Ex eriment p (equivs./l.)

Height (HETP in mm.)

EXAMPLE III The original adsorbed band consisted of 3 /2 liters of 15 Nammonium hydroxide (diluted 30-fold for loading). The band was eluteddown 30 columns with 0.6 N NaOH at a flow rate of 400 ml. per minutebefore any additional ammonia was added or any N rich product waswithdrawn. Thereafter, 550 ml. of 15 N ammonium hydroxide in 15 litersof solution were added each cycle and a corresponding amount of Nproduct was withdrawn. No enriched N product was removed, except whenprofiles of the adsorbed band were taken to determine its condition.Injections were initially made nine inches ahead of the tail-end of theband, but as the N concentration in the rear part of the band increased,it became necessary to gradually move the point of injection forward inorder to stay on the normal plateau. The injection point was 16 inchesfrom the tail-end of the band by the 500th column.

The depleted product being withdrawn early in the experiment had a N Nratio of about 0.00012, but the ratio tended to increase gradually. Whenthe N of 0.81 was observed at the'reari assacos N ratio of the depletedproduct exceeded 0.00050, the length of'tlie "liand"Was iiicreased'bymaking an addition but no Withdrawal. In 2511, two such increases weremade so 'thafthe band was about 100 inches long by colnnin number 550and contained the equivalent of 4.6 lifei's of 15 N ammonia or69ffnole's. 7

It is "apparent that these semi-continuously fed coluf'n'ns can beoperated under conditions where 99.99% N is withdrawn 'at the front andgreater than 99% N is ble'dslo'virly from the "rear edge of the band.

While in the foregoing specificationthis invention has been describedinrl'a'tion to specific er'nbodime'ntsthe'reof and many details havebeen set forth for purpose of illustration, it will be apparent to thoses'killed in the aftfthat the invention is susceptible to additional'einbodiments "and that many-of the details set forth herein can bevaried considerably 'witlfout departing from the basic principles of theinvention.

We claim:

1. The method of separating nitrogen isotopes by ionexchange,characterized 'by the steps o'f passin an aqueons-solutionof ammoniumhydroxide through a portion of a strong acid cation exchangeresin in thehydrogen cycle to provide an adsorbed animonium ban'd having asharply-defined front boundary, said resin being substantially-uniformlypacked inanlongate'cl column, said ammonium hydroxide providing both Nand N ammonium ions, thenehitin'g the adsorbed ammonia while displacingsaid band to successive portions of acation exchange material in thehydrogen cycle by passing an aqueous solution of an alkali-metalhydroxide through the cation exchange material behind said band, therebymaintainingasharply-defined rear boundary for saidband, and continuingsaideluting and displacing until the rear portion of said band containsan appreciablyhigher ratio of N ammonia to N ammonia than the frontportion of said hand said ammonium hydroxide solution haying a normalityranging from 0.01 normal to 3.0 normaland said alkali metal hydroxidesolution'having a normality ranging-from 0.01 normal to 3;0 normal.

2. The method of claim 1 in which said cation exchange material is asulfonated styrene-divinylbenzene copolymer, and in which said alkalimetal hydroxide is-sodiumfhydroxide. v

, 3. The method of claim 1 in which said eluting and displacing arecontinued until the rear -portion of said band contains at least molepercent N ammonia'on the basiscf the total ammonia.

4. The method of claim 1 in which-said alkali metal hydroxide is sodiumhydroxide.

References Cited in the-file of this patent UNITEDSTATES PATENTS OTHERREFERENCES Kunin and Myers'book, Ion Exchange Resins, pp. 26-3 1,JohnWiley and Sons, Inc, NY.

Burrell: Article inInd; and Eng. Chem, Vol.30, No. 3, March 1938, pages358-360.

1. THE METHOD OF SEPARATING NITROGEN ISOTOPES BY IONEXCHANGEMCHARACTERIZED BY THE STEPS OF PASSING AN AQUEOUS SOLUTION OF A,,ONIUMHYDROXIDE THROUGH A PORTION OF A STRONG ACID CATION EXCHANGE RESIN INTHE HYDROGEN CYCLE T PROVIDE AN ADSOPBED AMMONIUM BAND HAVING ASHARPLY-DEFINED FRONT BOUNDARY, SAID RESIN BEING SUBSTANTIALLY UNIFORMLYPACKED IN AN ELONGATED COLUMN, SAID AMMONIUM HYDROXIDE PROVIDING BOTHN15 AND N14 AMMONIUM IONS, THEN ELUTING THE ADSORBED AMMONIA WHILEDISPLACING SAID BAND TO SUCCESSIVE PORTIONS OF A CATION EXCHANGEMATERIAL IN THE HYGROGEN CYCLE BY PASSING AN AQUEOUS SOLUTION OF ANALKALI METAL HYDROXIDE THROUGH THE CATION EXCHANGE MATERIAL BAHIND SAIDBAND, THEREBY MAINATAINING A SHARPLY-DEFINED REAR BOUNDARY FOR SAIDBAND, AND CONTINUING SAID ELUTING AND DISPLACING UNTIL THE REAR PORTIONOF SAID BAND CONTAIN AN APPRECIABLY HIGHER RATIO OF N15 AMMONIA TO N14AMMONIA THAN THE FRONT PORTION OF SAID BAND, SAID AMMONIUM HYDROXIDESOLSUTION HAVING A NORMASLLY RANGING FROM 0.01 NORMAL TO 3.0 NORMAL ANDSAID ALKALI METAL HYDROXIDE SOLUSTION HAVING A NORMALLY RANGING FROM0.01 NORMAL TO 3.0 NORMAL.